1. Technical Field
The subject matter described here generally relates to fluid reaction surfaces with specific blade structures that are formed with a main spar, and, more particularly, to wind turbine blades having cross webs.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by the machinery, such as to pump water or to grind wheat then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is converted to electricity, then the machine may also be referred to as a wind generator or wind power plant.
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1 and available from General Electric Company. This particular configuration for a wind turbine 2 includes a tower 4 supporting a nacelle 6 enclosing a drive train 8. The blades 10 are arranged on a hub to form a “rotor” at one end of the drive train 8 outside of the nacelle 6. The rotating blades 10 drive a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 8 arranged inside the nacelle 6 along with a control system 16 that receives input from an anemometer 18.
The blades 10 generate lift and capture momentum from moving air that is them imparted to a rotor as the blades spin in the “rotor plane.” Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The distance from the tip to the root, at the opposite end of the blade, is called the “span.” The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade.
A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of the chord line is simply called “the chord.” Since many blades 10 change their chord over the span, the chord length is referred to as the “root chord,” near the root, and the “tip chord,” near the tip of the blade. The chord lines are arranged in the “chord planes” that extend through the streamlines on the corresponding pressure and suction surfaces of the blade. Multiple “shear web planes” are arranged perpendicular to the to the chord plane.
As illustrated in FIG. 2, the blades 10 for such wind turbines 2 are typically fabricated by securing various “shell” portions 20 to one or more “spar” members 22 such as with a bonding material 24. The spar members 22 extend at least partially spanwise along the inside of the blade 10 and are typically configured as I-shaped beams having a web referred to as a “shear web” 26 that typically extends between two flanges, referred to as “caps” or “spar caps” 28. For example, the spar caps 28 may be joined to the inside of the suction and pressure surfaces of the shell 20 or they may form part of the shell. However, the shear web 26 may also be utilized without caps and the various components of the shell 20 and/or spar 22 may be integrally formed without bonding materials 24. Other configurations may also be used for the spar 22 including, but not limited to “C-,” “L-,” “T-,” “X-,” “K-,” and/or box-shaped beams.
Spar cap buckling and torsional stability are just two of the problems associated with the design of blades 10. For example, the spar caps 28 must often be thickened, such as with additional layers of material, in order to prevent such buckling. However, this approach toward addressing the problem of spar cap buckling can significantly increase the weight of the blade 10 and does not necessarily provide significant improvements in torsional stability.